Dependence of tRNA Structure in Solution upon Ionic Condition of the Solvent

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Abstract
Dependence of barley phenylalanine tRNA (tRNAPhe) fluorescence intensity at 430 nm upon LiCl, NaCl, KCl, CsCl or NH4Cl concentration was measured in 0.01 M Tris-HCl, pH 7.5, 0.001 M Na2EDTA solutions. Increase of monovalent cation concentration in the solvent from 0-2 M induced about 3-fold fluorescence intensity enhancement. The fractional fluorescence change was used as a measure of bound ligand concentration. Fluorescence Scatchard plots revealed 3 classes of monovalent cation binding sites on the tRNA molecule: interacting (strong) and independent (weak and very weak) sites. Calculated from Scatchard plots binding constants (K), for strong and weak binding of monovalent cations (in the case of Na+ binding: KS [strong binding constant] = 26 M-1 and Kw [weak binding constant] = 4.3 M-1, respectively) exhibit linear dependence upon ionic radius (r). Two limiting values obtained from the plot of K vs. r: K(max) at r = 0 and r(max) = 42 M-1, KW(max) = 8.5 M-1, rs(max) = 0.23 nm and rw(max) = 0.22 nm). A model of the relationship between weak Mg2+ binding sites and monovalent cation binding sites as well as of monovalent cations binding to tRNA is proposed.